A jack-up rig is an offshore oil exploration drilling structure for use in shallow water, typically in water depths up to 500 feet. The jack-up rig normally comprises a floatable hull with a deck or working platform and three or four legs. The legs pass through leg openings in the hull and are moveable relative to the hull. After the jack-up rig has arrived on location, the legs of the jack-up rig are lowered until they touch a seabed beneath the hull. This allows the hull to be supported by the legs that rest on a foundation soil on the seabed so that the hull may be jacked up using a jacking system to raise the working platform above the water, making the jack-up rig safer to be operated in open water situations where water movement is experienced.
The legs of a jack-up rig are commonly trusses, each truss comprising a number of vertical chords interconnected with cross braces. A leg typically comprises three or four chords. The legs normally terminate in a footing that rests on the seabed. The footing provides an enlarged soil bearing area so as to reduce pressure exerted on the soil of the seabed. This in turn reduces bearing capacity that is required by the soil to support the jack-up rig, allowing the jack-up rig to be operated in a greater variety of locations and soil types.
To move the legs relative to the hull, a rack-and-pinion mechanism is normally used. Each vertical chord of each leg is typically provided with two longitudinally aligned racks disposed diametrically on either side of the chord, with the teeth of each rack directed laterally away from the chord. Pinions to engage the racks are provided on jack-cases that are installed in the leg openings in the hull through which the legs pass. Rotation of the pinions on the jack-cases against the racks on the chords of each leg thus moves the leg relative to the hull. To keep the legs aligned as they pass through the leg openings in the hull, guides are provided in the jack-cases for sliding engagement with the racks on the chords of the legs as the legs are moved relative to the hull.
For offshore oil exploration, the jack-up rig is normally brought to site and installed there for months at a stretch. This means that relative movement of the legs occurs only every few months at most, that is, when the jack-up rig arrives at site and is installed, and when the exploration operation is completed months afterwards and the jack-up rig leaves the site.
For installing structures such as offshore wind turbines, a jack-up barge that operates similarly to jack-up rigs is often used. Installing one wind turbine from an installation vessel such as a jack-up barge typically takes only a few days. An offshore wind farm can comprise scores to several hundred individual wind turbines. Installing an entire offshore wind farm therefore requires the installation vessel to be moved from one designated wind turbine site to the next designated wind turbine site every few days, several hundred times over.
Moving a jack-up barge every few days several hundred times means that legs of the jack-up barge have to be raised and lowered relative to the hull at least an equal number of times. Guides currently used in oil exploration jack-up rigs are therefore not suitable for use in installation vessels such as jack-up barges because they are designed only to slideably engage the racks on the legs at most a few times every few months, and are consequently unable to withstand the wear from sliding against the racks every few days many times over, as would be required when installing offshore wind turbines.